Lecithin cholesterol acyltransferase (LCAT) is a 63 kD enzyme that is responsible for the esterification of virtually all free cholesterol in plasma, Cholesteryl esters formed by LCAT are incorporated preferentially into HDL particles. This pathway is considered to play a central role in reverse cholesterol transport. Patients with functional LCAT defects can present with remarkable different clinical features, based on either a partial enzyme deficiency which has been designated Fish Eye Disease (FED) or a total enzyme deficiency designated classical LCAT deficiency. We have identified the underlying molecular defects in the LCAT gene of two patients presented with the two different types of LCAT defects. The proband from the first kindred is a 66 yo male of German descent with FED presenting with corneal opacities, an HDL-cholesterol level of 7.7 mg/dl (nl=35-65), virtually absence of LCAT activity using a proteoliposome substrate and normal cholesterol esterification rate. DNA sequence analysis of the patient's gene revealed two separate C to T transitions resulting in the substitution of Thr-123 by IIe and Thr-347 by Met. The mutation at codon 347 created a new restriction site for Nla III. Analysis of patient PCR amplified DNA by digestion with Nla III established that the proband is a compound heterozygote for both mutations, These studies indicate that the regions adjacent to Thr-123 and Thr-347 of LCAT may play an important role in the ability of the enzyme to esterify cholesterol present in HDL. While tyr-156 is necessary for esterification of cholesterol present in any lipoprotein particle, these studies will provide insight into important functional domains of LCAT required for cholesterol esterification. The proband from the second kindred is a 52 yo white male American presenting with clinical features of classic LCAT clinical features including cloudy corneas, a mild proteinuria normochromic anemia and an HDL-cholesterol of 7.7 mg/dl. Plasma cholesterol esterification rate and LCAT activity were undetectable. DNA sequence analysis of the patient's LCAT gene revealed two separate point mutations resulting in the introduction of a stop codon at residue 83 and the substitution of Tyr- 156 by Asn. These mutation s result in the loss of restriction sites for the enzymes Accl and Rsal which were used to establish that the proband is a compound heterozygote for both defects. These data suggest that residue 156 is essential for normal LCAT function.